ORIGIN OF THE LAND VERTEBRATES 99 



heart. There cannot be any doubt therefore that 

 the air-bladder of the Dipnoi is the homologous fore- 

 runner of the land Vertebrate's lung, and that its 



d. 



Fig. 20, B and D. Diagrams illustrating the blood supply of the air- 

 bladder in B, Ceratodus, and D, a Teleost. The blood-vessels are 

 seen from behind and are cut short in transverse section, a. dorsal 

 aorta; aad. anterior dorsal artery from the coeliac; aav. anterior 

 ventral artery ; ab. air-bladder; avd. anterior dorsal vein to the 

 cardinal; &a 6 , 4th aortic arch; cv. coeliac artery; d. ductus 

 Cuvieri ; la. left pulmonary artery ; oe. oesophagus ; pr. portal 

 vein; rpv. right pulmonary vein; v. left pulmonary vein. (After 

 E. S. Goodrich, Lankester's Treatise.) 



function is already the same, namely for the supply 

 of atmospheric oxygen to the blood. 



The habits and nature of the Dipnoi are of such 

 interest that we may describe them in some detail. 

 There are three species of Dipnoi or Lung-fishes 



72 



